In recent years, there has been an increasing need for measuring a single molecule (for example, protein) in a cell and clarifying the mechanism of diseases or life phenomena in the bio field centered on life sciences. In order to satisfy such a need, an ultrasensitive analysis technique, with which cells can be observed alive and unlabeled, has become indispensable.
Currently, as a detecting method in the bio field, surface enhanced Raman spectroscopy is gaining attention as an ultrasensitive analyzing method which combines “spectroscopy using the Raman effect” and “the enhancing effect of light on a metal surface,” and is used to identify substances and the like. The Raman effect refers to a phenomenon in which, when light enters a substance, the scattered light includes light having different wavelengths from the wavelength of the incident light (inelastic scattering). The scattered light at this time is called Raman scattered light. Raman spectroscopy is known in which, since the difference in energy between the light scattered by the Raman effect and the incident light corresponds to the energy at the vibration level, rotation level, or electron level of a molecule or crystal in a substance, and the molecule or crystal has an intrinsic vibration energy in accordance with the structure, chemical species are identified from spectra, and the quantity of a target substance is determined from the intensity of the scattered light using a phenomenon in which the molecule or crystal is modulated to light in which the molecular intrinsic energy state is reflected, by using a laser which is a monochromatic light. However, since the sensitivity of Raman spectroscopy is intrinsically low, the Raman spectroscopy is not appropriate for analysis of a small amount of specimen.
On the other hand, in a metal nano-particle, plasmon, which, is a phenomenon in which free electrons present on a metal surface collectively vibrate, occurs on the metal surface, and this surface plasmon is coupled with an optical electric field in a visible light to near-infrared region, thereby significantly enhancing the electric field on the surface of the metal nano-particle. Surface enhanced Raman spectroscopy has come to gain attention since use of this surface plasmon resonance irradiates laser light to a molecule adsorbed to the surface of the metal nano-particle, and drastically enhances the Raman scattered light that is generated from the adsorbed molecule. One of the surface enhanced Raman spectroscopies that are carried out is SERS measurement in which the fact that a substance is adsorbed to the surface of a noble metal electrode, such as gold and silver, or colloid, and the vibration spectrum is enhanced in comparison to a single molecule is used (PTL 1).
This SERS measurement is a useful method for the structural analysis of a small amount of a substance; however, currently, this method is told that it is necessary to accumulate fine particles of a noble metal, such as silver and gold, having a size of about several tens of nm to several hundreds of nm on a glass substrate, and, in the past, it was necessary to synthesize colloid particles of silver or gold in solution, and fix them on a substrate modified by a lysine or cyan (NPL 1, 2, and 3, and PTL 2). Particularly, in PTL 2, a so-called drop & dry method is employed in which colloids that have been prevented from agglomerating are gelatinized, coated, and dried so as to produce a substrate, and form a main stream.